This invention relates generally to fasteners for securing composite decking to a steel supporting structure, and more particularly to a self-drilling fastener for use as a self-drilling stud in lightweight or normal weight composite deck structures which are used as floors or roofs in modern buildings.
During construction of a composite deck, sheets of metal decking, which is often corrugated but can also be flat, are usually fastened to steel structural members. Located along and projecting upwardly from the steel members are metal studs. A concrete slab formed over the steel decking encapsulates the metal studs, so that the studs restrict relative shear movement between the concrete slab and the steel member.
Two general types of studs, weld studs and self-drilling studs, are typically used in a composite deck. Standard weld studs are welded directly to the decking and steel structural members. These studs are ductile so they are suited to restrict the relative shear movement between the concrete slab and the steel. Installation of a weld stud requires that it burn through the decking in order to attach itself to the steel support member. During this process the decking will burn away around the steel weld stud and will typically not attach itself to the support member. This would require a separate attachment operation to attach the deck to the support member.
Further a round ceramic insulator is used at the end of the weld stud during the installation process. it is used in order to concentrate the heat and assist in welding of the stud to the support member. Once the weld stud is attached, the round ceramic insulator needs to be removed from the base of the weld stud, in order for the concrete to encapsulate the stud. At this point the typical way of removing these ceramic insulators is to strike them with a hammer to break them away from the base of the installed weld studs. Since these ceramic insulators are brittle, when broken away from the weld studs they shatter and scatter ceramic pieces over the deck surface. These ceramic pieces need to be removed from the deck surface prior to concrete pour, both for safety reasons and to avoid integration into the concrete mixture and contamination of the concrete integrity. Use of self-drill shear studs does not require the above extra operations.
In the process of stud welding through the decking the protective deck coating is damaged from the intense heat produced, rendering it susceptible to corrosion. Typical recommended practice is to apply a protective coating to the bare or damaged decking material to reduce corrosion effects. Use of self-drill shear studs does not require the above extra operation.
Due to the nature of the weld joint between the stud and the steel, the diameter of the weld stud establishes a minimum material thickness that is required for the structural member. The smallest weld studs that are presently commercially used establish a lower limit on the material thickness of the structural member that can be used with those weld studs. The smallest diameter weld stud currently available is 1/2" diameter which requires a minimum of 0.200" thick steel. If steel thicknessies less than 0.200" usually encountered in typical bar joist construction, the weld stud is not recommended. Self-drilling shear studs present an alternative in these applications.
Self-drilling studs are thus an attractive alternative to weld studs for use in composite decks. Self-drilling studs do not limit the minimum material thickness of the structural member. In addition, self-drilling studs secure the decking to the steel, thereby eliminating the separate attachment needed with the use of weld studs. Self-drilling studs also avoid other problems with weld studs, such as the removal of the ceramic insulator from the stud after its installation, and the cleanup of ceramic pieces which are occasioned by the removal. The decking is not damaged, and indeed, the need to repair the metal decking and attach it to the stud or the metal support is eliminated. Use of a self-drilling stud does not create a damaged area of the decking which must be repaired and instead, when properly installed provides a means for attaching the deck to the steel member. However, in order for the self-drilling studs to be capable of drilling the steel, the self-drilling studs must be heat treated to a relatively high degree of hardness. This standard processing results in the reduced ductility, and ductility is necessary in the self-drilling stud to transfer the shearing movement in the composite deck system. Some self-drilling studs have attempted to compensate for this basic deficiency by isolating the stud from the concrete with a spacer. The spacer attempts to convert the shearing action of the concrete slab into a bending moment which the hardened stud is more adept at resisting. However, it does not change the non-ductile nature of the self-drilling studs themselves.